Infectious diseases are the leading cause of death worldwide. According to a 1999 World Health Organization study, over 13 million people die from infectious diseases each year. Infectious diseases are the third leading cause of death in North America, accounting for 20% of deaths annually and increasing by 50% since 1980. The success of many medical and surgical treatments also hinges on the control of infectious diseases. The discovery and use of antibiotics has been one of the great achievements of modern medicine. Without antibiotics, physicians would be unable to perform complex surgery, chemotherapy or most medical interventions such as catheterization.
Current sales of antibiotics are US$26 billion worldwide. However, the overuse and sometimes unwarranted use of antibiotics have resulted in the evolution of new antibiotic-resistant strains of bacteria. Antibiotic resistance has become part of the medical landscape. Bacteria such as vancomycin-resistant Enterococcus, VRE, and methicillin-resistant Staphylococcus aureus and MRSA, strains cannot be treated with antibiotics and often, patients suffering from infections with such bacteria die. Antibiotic discovery has proven to be one of the most difficult areas for new drug development and many large pharmaceutical companies have cut back or completely halted their antibiotic development programs. However, with the dramatic rise of antibiotic resistance, including the emergence of untreatable infections, there is a clear unmet medical need for novel types of anti-microbial therapies, and agents that impact on innate immunity would be one such class of agents.
The innate immune system is a highly effective and evolved general defense system. Elements of innate immunity are always present at low levels and are activated very rapidly when stimulated. Stimulation can include interaction of bacterial signaling molecules with pattern recognition receptors on the surface of the body's cells or other mechanisms of disease. Every day, humans are exposed to tens of thousands of potential pathogenic microorganisms through the food and water we ingest, the air we breathe and the surfaces, pets and people that we touch. The innate immune system acts to prevent these pathogens from causing disease. The innate immune system differs from so-called adaptive immunity (which includes antibodies and antigen-specific B- and T-lymphocytes) because it is always present, effective immediately, and relatively non-specific for any given pathogen. The adaptive immune system requires amplification of specific recognition elements and thus takes days to weeks to respond. Even when adaptive immunity is pre-stimulated by vaccination, it may take three days or more to respond to a pathogen whereas innate immunity is immediately or rapidly (hours) available. Innate immunity involves a variety of effector functions including phagocytic cells, complement, etc, but is generally incompletely understood. Generally speaking many innate immune responses are “triggered” by the binding of microbial signaling molecules with pattern recognition receptors termed Toll-like receptors on the surface of host cells. Many of these effector functions are grouped together in the inflammatory response. However too severe an inflammatory response can result in responses that are harmful to the body, and in an extreme case sepsis and potentially death can occur.
The release of structural components from infectious agents during infection causes an inflammatory response, which when unchecked can lead to the potentially lethal condition, sepsis. Sepsis occurs in approximately 780,000 patients in North America annually. Sepsis may develop as a result of infections acquired in the community such as pneumonia, or it may be a complication of the treatment of trauma, cancer or major surgery. Severe sepsis occurs when the body is overwhelmed by the inflammatory response and body organs begin to fail. Up to 120,000 deaths occur annually in the United Stated due to sepsis. Sepsis may also involve pathogenic microorganisms or toxins in the blood (e.g., septicemia), which is a leading cause of death among humans. Gram-negative bacteria are the organisms most commonly associated with such diseases. However, gram-positive bacteria are an increasing cause of infections. Gram-negative and Gram-positive bacteria and their components can all cause sepsis.
The presence of microbial components induce the release of pro-inflammatory cytokines of which tumor necrosis factor-α (TNF-α) is of extreme importance. TNF-α and other pro-inflammatory cytokines can then cause the release of other pro-inflammatory mediators and lead to an inflammatory cascade. Gram-negative sepsis is usually caused by the release of the bacterial outer membrane component, lipopolysaccharide (LPS; also referred to as endotoxin). Endotoxin in the blood, called endotoxemia comes primarily from a bacterial infection, and may be released during treatment with antibiotics. Gram-positive sepsis can be caused by the release of bacterial cell wall components such as lipoteichoic acid (LTA), peptidoglycan (PG), rhamnose-glucose polymers made by Streptococci, or capsular polysaccharides made by Staphylococci. Bacterial or other non-mammalian DNA that, unlike mammalian DNA, frequently contains unmethylated cytosine-guanosine dimers (CpG DNA) has also been shown to induce septic conditions including the production of TNF-α. Mammalian DNA contains CpG dinucleotides at a much lower frequency, often in a methylated form. In addition to their natural release during bacterial infections, antibiotic treatment can also cause release of the bacterial cell wall components LPS and LTA and probably also bacterial DNA. This can then hinder recovery from infection or even cause sepsis.
Cationic peptides are being increasingly recognized as a form of defense against infection, although the major effects recognized in the scientific and patent literature are the antimicrobial effects (Hancock, R. E. W., and R. Lehrer. 1998. Cationic peptides: a new source of antibiotics. Trends in Biotechnology 16:82-88.). Cationic peptides having antimicrobial activity have been isolated from a wide variety of organisms. In nature, such peptides provide a defense mechanism against microorganisms such as bacteria and yeast. Generally, these cationic peptides are thought to exert their antimicrobial activity on bacteria by interacting with the cytoplasmic membrane, and in most cases, forming channels or lesions. In gram-negative bacteria, they interact with LPS to permeabilize the outer membrane, leading to self promoted uptake across the outer membrane and access to the cytoplasmic membrane. Examples of cationic antimicrobial peptides include indolicidin, defensins, cecropins, and magainins.
Recently it has been increasingly recognized that such peptides are effectors in other aspects of innate immunity (Hancock, R. E. W. and G. Diamond. 2000. The role of cationic peptides in innate host defenses. Trends in Microbiology 8:402-410; Hancock, R. E. W. 2001. Cationic peptides: effectors in innate immunity and novel antimicrobials. Lancet Infectious Diseases 1:156-164) although it was not known if the antimicrobial and effector functions are independent.
Some cationic peptides have an affinity for binding bacterial products such as LPS and LTA. Such cationic peptides can suppress cytokine production in response to LPS, and to varying extents can prevent lethal shock. However it has not been proven as to whether such effects are due to binding of the peptides to LPS and LTA, or due to a direct interaction of the peptides with host cells. Cationic peptides are induced, in response to challenge by microbes or microbial signaling molecules like LPS, by a regulatory pathway similar to that used by the mammalian immune system (involving Toll receptors and the transcription factor; NFκB). Cationic peptides therefore appear to have a key role in innate immunity. Mutations that affect the induction of antibacterial peptides can reduce survival in response to bacterial challenge. As well, mutations of the Toll pathway of Drosophila that lead to decreased antifungal peptide expression result in increased susceptibility to lethal fungal infections. In humans, patients with specific granule deficiency syndrome, completely lacking in α-defensins, suffer from frequent and severe bacterial infections. Other evidence includes the inducibility of some peptides by infectious agents, and the very high concentrations that have been recorded at sites of inflammation. Cationic peptides may also regulate cell migration, to promote the ability of leukocytes to combat bacterial infections. For example, two human β-defensin peptides, HNP-1 and HNP-2, have been indicated to have direct chemotactic activity for murine and human T cells and monocytes, and human β-defensins appear to act as chemoattractants for immature dendritic cells and memory T cells through interaction with CCR6. Similarly, the porcine cationic peptide, PR-39 was found to be chemotactic for neutrophils. It is unclear however as to whether peptides of different structures and compositions share these properties.
The single known cathelicidin from humans, LL-37, is produced by myeloid precursors, testis, human keratinocytes during inflammatory disorders and airway epithelium. The characteristic feature of cathelicidin peptides is a high level of sequence identity at the N-terminus prepro regions termed the cathelin domain. Cathelicidin peptides are stored as inactive propeptide precursors that, upon stimulation, are processed into active peptides.